The long-term goal of this research is 1) to understand mechanisms used by typically developing (TD) infants in attaining head & trunk control for the development of sitting balance and associated reaching skills, and 2) to apply that knowledge to understanding and improving these skills in children with cerebral palsy (CP). While trunk postural control creates the basis for reaching skills, little is known about how spinal stability develops; in fact there are almost no clinical measures of intermediate stages of trunk stability. In the case of children with CP this lack of knowledge is critical. Our experiments address these limitations. Experiments will determine developmental changes in kinematics of head stability, trunk muscle activation patterns and patterns of reaching movements with different levels of trunk support in TD infants and children with CP. Specific Aim 1 is a longitudinal study evaluating contributions of spinal segments to head stability and reaching skills in 3-9 month old TD infants as they gain sitting balance. We hypothesize: 1) head stability emerges first at higher levels of trunk support and stability progresses sequentially down the spine in a time dependent manner, associated with the rostro-caudal development of coordinated muscle response patterns during sitting; and 2) emergence of efficient reaching movements is associated with the mastery of postural control at a given level of trunk development. Specific Aim 2 is a cross-sectional study evaluating contributions of spinal segments to trunk stability and reaching in children with moderate to severe CP. We hypothesize that 1) within children with various levels of severity of CP (Gross Motor Function Classification System levels III, IV, V) we will see arrested levels of spinal development along a continuum of that which is seen during sitting balance development in TD children; thus lack of sitting spinal control reflects deficits in coordination at a specific spinal level; 2) reaching deficits will be associated with postural control deficits at specific spinal levels. The study of segmental contributions to spinal control is novel and challenges current clinical concepts for assessment and treatment of sitting development; thus it brings critical new information to our understanding of both typical development and constrained development in children with CP.